Signaling system



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SIGNALING SYSTEM Filed April 23, 1941 l0 Sheets-Sheet 5 W diffu-ATTORNEY Nov. 17, 1942. E. R. TAYLQR` SIGNALING SYSTEM Filed April 23,1941 l0 Sheets-Sheet 6 E. R. TAYLOR AHORA/5V Nov. 17, 1942. E. R. TAYLORSIGNALING SYSTEM Filed April 25. 1941 l0 Sheets-Sheet '7 /A/l/EA/TOR E.TAYLOR 5V A r TURA/EV NOV. 17, 1.942. E, R TAYLOR] 2,302,071

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SIGNLING SYSTEM Filed April 2s. 1941 1o sheets-sheet 1o E R. mno/P `M 9.ima

ATTORNEY Patented Nov. 17, 1942 recaen SIGNALING SYSTEM Edmund R.Taylor, Mount Vernon, N. Y., assignor to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York ApplicationApril 23, 1941, Serial No. 389,836

30 Claims.

This invention relates to signaling systems and particularly totelephone systems in which voice frequency currents are used fortransmitting signals.

Objects of the invention are the provision of more reliable signaltransmitting and receiving means in systems employing Voice frequencysignaling, the provision of a voice frequency signal receiver adapted toquickly respond to seizure of the toll line with which the receiver isassociated, and the prevention of false operations in response to voicecurrents or other interfering currents.

This invention is an improved signaling systern comprising means fortransmitting signals comprising current of a first frequency followed bycurrent of a second frequency and signals comprising current of thesecond frequency followed by current cf the rst frequency, and arrangedtorespond to such signals without responding to voice currents or otherinterfering currents. imilar signal transmitting and receiving means areconnected to each end of a toll telephone line, different pairs offrequencies being used for signaling in each direction thereover.

According to one feature of the invention, the signal receiver isarranged to recognize current of one of two signaling frequencies as aseizure signal to effect a busy marking of the toll line without waiting'for current of the other of the two frequencies and thereafter toeffect a signal response in response to current of either of the twofrequencies only when immediately preceded by current of the other ofthe two frequencies.

According to another feature of the invention, a disconnect signalcomprises current of the second of the signaling frequencies followed bycurrent of the rst of the two frequencies, the signal transmitter beingarranged to repeat this signal to make certain that release of anestablished connection is effected. The signal receiver is arranged toeffect release in response to the first of the two disconnect signalsreceived over the line with which the receiver is associated. If thesignal receiver fails to recognize the first disconnect signal, due forinstance to interference currents in the line, it may recognize thesecond of the two disconnect signals.

In either case, the busy marking of the toll line with which the signaltransmitter and receiver are associated is maintained at both ends ofthe toll line until the second disconnect signal 'is ended.

A further feature of the invention is the pro?v vision of means in asignal receiver adapted to respond to signals consisting of a sequenceof currents of two different frequencies for preventing a signalresponse until the current of the second of the two frequencies hascontinued for a predetermined interval of time, thereby preventing asignal operation in response to signaling currents transmitted throughthe talking connection in a tandem office on connections between twolines arranged for transmitting voice frequency signals of similarcharacter.

A further feature of the invention is the provision of separate volumelimiting amplifiers in a signal receiver adapted to respond to signalsconsisting of a sequence of currents of two different frequencies, onefor each frequency, to limit the current applied to each ofthe signalingrelays independent of variations of the signaling current over the line,thereby reducing signal distortion. Band-pass lters are inserted'between the amplifiers and the line, one for each frequency, so as toreduce the possibility of operation of the signal relays by voice andnoise currents; and the limiters are of the slow recovery type so as toprevent echoes of the first frequency from interfering with the releaseof the relay responsive to the rst current impulse of a signal andoperation of the relay responsive to the second current impulse of thesignal.

A clear and complete understanding of the invention will be facilitatedby considering a system embodying the invention and its Variousfeatures, one such system being represented schematically in thedrawings which form a part of this specification. The invention is notlimited in its application to the particular system and circuitarrangements shown in the drawings but is applicable, generally, to anyvoice frequency signaling or telephone system.

Referring to the drawings:

Figs. l to 3, inclusive, show in a first toll o'ice an outgoing trunkcircuit OTI, a two-way trunk circuit 'I'WT2, an incoming trunk circuitITI,

a toll line circuit TLCI, a signal transmitter ST4, and a signalreceiver SR! all associated with a toil line T12;

Figs. e to 6, inclusive, show in a tandem toll office a toll linecircuit TLCZ, signal receiver SR2, signal transmitter ST2, two-way trunkcircuit TWIE, outgoing trunk circuit 0T2, and a selector TS2, allassociated with toll line Ti, and show an incoming trunk circuit ITE;

Figs. '7 to 9, inclusive, show a toll line circuit Til-C3, signalreceiver SH3, signal transmitter ST3;

two-way trunk circuit TWT3, outgoing trunk circuit OTS and selector TS3all associated with a toll line T36;

Fig. l shows signal receivers SRI and SR2 connected to the ends of tollline TIZ, each of the signal receivers SRi and SR2 being modificationsof the signal receivers SRS and SR2 of Figs. 3 and 4; and

Fig. 11 shows the relative position in which Figs. 1 to 9, inclusive,are to be placed to form an operative arrangement.

Reference may be had to the patent to R. King et al. No. 2,209,777,granted July 3G, 1940, for a detailed disclosure of the cord andoperators position circuits provided in each of the toll offices forinterconnecting and controlling the completion of toll calls. Theoutgoing trunk circuits OTI, 0T2 and OT3, incoming trunk circuitsITI andIT2, two-way trunk circuits TWTI, TWT2 and TWT3 and the toll routeselector circuits TS2 and TSS are all similar'to correspondingu circuitsin the aforementioned King et` al. patent and also-to correspondingcircuits of the copending application of J. G. Walsh, Ser. No.- 389,835,filed April 23, 1941. The toll line-circuits TLCI, TLCZ and TLC3 and theassociated sig-nal transmitting and receiving circuits STI, ST2, ST3,SRI, SR2 and SR3 are arranged for voice frequency signaling over theassociatedy toll lines; whereas in the aforemeno tioned King et al;patent composite signaling circuits are associated with the toll lines.

The toll line circuit TLCI comprises a repeating coil 3D0; adirectionallyselective coil 3&3, a sending cut-01T" relay 240 fordisconnecting the talking conductors I6! and |62 of the associated trunkcircuits from coil 300 when signals are being transmitted over line T|2to the tandem oil-ice, a repeater termination consisting of condenser`I- andresistor252 for connection across the talking conductors whilesignals are being transmitted to or received from the tandem offlce, atransformer 239 for impressing the signaling potentials acrossconductors 2M and 2922, and a receiving cut-off relay 25|)Vfor-disconnecting the talking conductors I 5| and |52 from coil `whilesignalsare incoming overV line TI2 from-the tandem office. Signalingcurrents incoming over toll line T|2 pass through the directionallyselective coil V3li3 to the signal receiver SRI; Signaling currentsoutgoing over toll line TI2- from transmitter STI also pass throughcoil303,- the coil'and associated resistors Iiil and 302 preventing anysubstantial portion of4 the signaling currents outgoing from transmitterST| from passing into the signal receiver SRI.V EachV of the toll linecircuits TLC2 and TLC3 is similar to the line circuit TLCi, thereference'characters-of like elements having the same tens and unitsdigits.

The signal transmitter STI comprises a relay 20|) which receivesdirect-current signals which aretobe converted into voice frequencysignals for-transmission over line TI 2, relays 29|, 253, 2M, 29S,2|3,.2|5, 216 and 2i3 for effecting the transmission. of signalsconsisting of an impulse of current of frequency fl followed by currentof frequency f2. and signals consisting of current frequency f2 followedby current of frequency fl. The transmitter STI further comprisessources of signaling current 23| and 232, a transformer or repeatingcoil 239, busyA relays 22) and 22E, and various resistors andcondensers. A seizure signal sent by transmitterI STI on an outgoingcall and an answering supervisory or OFF-HOGE.

signal sent by transmitter STI on an incoming call consist of current offrequency fi for about .020 second followed by current of frequency f2for about .100 second; dial impulses consist of current of frequency f2for about .030 second followed by current of frequency fi for theduration of the impulse; an ON-HOOK signal and a rering signal are eachsimilar to a single dial impulse; and a disconnect signal sent bytransmitter STE consists of current of frequency f2 for about .G20second followed successively' by current of frequency fi for about .600second, current of frequency f2 for about .100 second, and current offrequency fl for about 1.3 seconds. Each of the signal transmitters ST2and ST3 is similar to the transmitter STi, the reference characters oflike elements having the same tens and units digits. Since differentfrequencies are used for signaling in opposite directions, the sources23! and 232 of transmitter STI generate frequencies fI and f2,respectively, and the sources 53| and-532 of transmitter ST2generatefrequencies f3 and f4, respectively. For instance, frequencies680 cycles and 1615 cycles may be used for transmitting signals overtoll line TIZ to the tandem office and frequencies 850 cycles and 1870cycles may be used for transmitting signals outgoing over toll line T|2from the tandem cnice. The frequencies used for signaling over toll lineT34fmay be similar to or different from the frequencies used for`signaling over line T|2. As shown in Fig. 8, the sourcesl 83| and S32 oftransmitter ST3 also generate potentials of frequencies fI and f2,respectively.

The signal receiver SRE comprises an amplifier 364, a filter 305 forattenuating noise, local plant tones and the signaling frequencies fland f2 used for transmitting signals outgoing over toll line TS2, avolume limiting amplifier 36S, a filter 3E? tuned to pass current offrequency f3, a filter SGS tuned to pass current cf frequency f4,amplifier detectors iii and Sli! and signal relays SII and 3Y|2 forresponding izo-signaling current of frequencies f3 and f, respectively.The output of the volume limiter 33S is somewhat in excess of thecurrent needed for operating each of relays 3 |-I and 3112 but isinsufficient to effect the operation of either of these relays unlesssubstantially all of the current is of frequency f3 or substantially allof frequency f4. Although the directionally selective coil 33 allowsonly a small portion of the signal energy from transmitter STI to reachthe signal receiver SRI, further protection against response to outgoingsignaling currents is provided by elements of filter 3125 tuned toattenuate the energy of frequencies JI and f2. Relays 3M, 355, 353, 32|,325, 326, 327.', 33|, 332, 333, 335, 335, 331 and 339 are controlled byrelays 3|| and 312 to effect desired closures and openings of a directcurrent signaling circuit over conductor if@ in response to incomingsignals consisting of current of frequency f3 followed by current offrequency ffl or consisting of current of frequency f6 followed bycurrent of frequency f3. Each of the signal receiving circuits SPA andSRS is similar to the receiver SRI, the reference characters of likeelements having the same tens and units digits. The receiver SR2 isarranged to Vrespond only to signals consisting of current of frequencyfl followed by current of frequency )"2 or to signals consisting ofcurrent of frequency f2 followed by current of frequency fI. The signalreceiver SR3 is arranged to respond only to signals Yconsisting ofcurrent of frequency f3 followed by current of frequency f4 or tosignals consisting of current of frequency f4 followed by current offrequency f3.

Further explanation of the invention will be facilitated by describingin detail the circuit operations involved in calls in both directionsover toll line T|2. Assume first that an operator in the first tolloflice inserts the plug P2 of a cord CDI in jack J2 on a call outgoingover toll line T|2. Relay |04 is operated by the connection of batteryto the sleeve conductor of jack J2 and relay |40 is operated, asdescribed in the aforementioned patent of R. E. King et al. Theoperation of relay I4!) closes a circuit from battery through resistor|42 and conductor |54 for operating relay 250 of the signal transmitterSTL Relay D opens the normally closed short circuit around relay 2|8,closes a circuit for operating busy relay 22|, closes a short circuitaround the winding of relay 23| and closes circuits including the innerfront Contact of relay 22| and windings of relays 23,3 and 24o. Relay 2I8 is slow in operating, closing its contacts within .O50 second afterrelay 2619 operates. Relay 20| is slow in releasing and remains operatedfor about .1GO second during which interval relays 253 and 249 operate.The operation of relay 22| connects ground to conductor 241 to operate abusy indicator-249 associated with line T|2 and closes a circuit foroperating relay 229 in series with relay 335 of the signal receiver SRI.Relay 335 also operates and closes a locking circuit which isindependent of relay 33E. Relay 220 disconnects the winding of relay 2|8from the back contact of relay 23) and connects this Contact to thewinding of the normally operated relay 2|5. The operation of relay 249disconnects the talking conductors |6| and |62 of trunk circuit OTI fromconductors 24| and 242 and coil 35), bridges the repeater terminationcomprising condenser 255i and resistor 252 across conductors 5| and |52,and connects the right winding of transformer 239 to and throughconductors 24| and 2552 to coil 302. The signaling sources 23| and 232are normally connected through conductors 235 and 235 and back contactsof relays 2&1 and 2|3, to ground at the haci: contacts of relays 233 and293, thereby short-circuiting the left winding of coil 239. When relay2&3 operates, it disconnects CilA ground from conductor 235 therebyinitiating current of frequency fl through the left winding of coil 239and closes a circuit for energizing the winding of relay Ztl. Relay 251is slow in operating; and, during an interval of about .O20 secondbefore relay 2i31 operates, the first part of a seizure signal istransmitted over toll line T|2.

When relay 251 operates, it connects ground to conductor to end thesignaling current of frequency f4 and disconnects ground from conductor235 thereby initiating current of frequency f2 through coils 2.39 andand line TS2. When relay 2|3 closes its contacts, within .O50 secondafter relay 293 operates, it connects the winding of relay 222 to theback contact of relay 2Q!) and connects ground at the outer frontcontact of relay 251, through front contacts of relays 23| and 2|8, andthrough both back contacts of relay 2|3 to conductor 235, in parallelwith the path to ground at the back contact of relay When relay 20|releases, about .1 second after relay 2%3 operates, the operatingcircuits of relays 223 and 240 are opened. The release of relay 293opens the operating circuit cf relay 231 and connects ground through itsback contact and inner back contact of relay 253 to conductor 235 toprevent current of frequency ,fl in coil 239 after relay 21 releases.Relay 221 is held operated, for an interval of about .020 second afterrelay 263 releases, by current charging condenser 265. Relay 240 beingslow in releasing, its contacts are not restored to normal for about.020 second after relay 28| releases. The release of relay 251reconnects conductor 23@ to ground at the back contact of relay 239; andthe release of relay 240 disconnects coil 239 from, and reconnectstalking conductors and |52 to, coil 3% and line T|2. rFhus the currentof frequency f2 in coil `Sill! and line T|2 continues for about .lsecond, that is, until relay 201 releases, or until relay 24d releasesif relay 24|) releases before relay 281 releases.

The seizure signal, thus transmitted over line T 2 to the signalreceiver SR2 in the tandem office, is passed through coil ll, the leftwindings of directionaliy selective coil 43, amplifier 484, and lter 495to volume limiting amplifier 325. rhe energy of frequency fl is furthertransmitted through filter 451 and ampli-ner detector 4129 to energizethe winding of relay 4| and the energy of frequency f2 is transmittedthrough iilter 4&8 and amplifier detector 4H! to energize the left,operating winding of relay 4t2. The amplitude of the output of amplifier4% is limited to a value sucient for operating relay 4|| only whensubstantially al1 of the current is of frequency fl and for operatingrelay 4|2 only when substantially all of `the current is of frequencyf2. Thus relays 4 and M2 will be operated in succession in response to aseizure signal; but these relays will seldom or never operate inresponse to noise, voice or other interfering currents having acomponent of frequency fl or frequency f2. The operation of relay 4| lcloses circuits including the lower winding of relay 4M, back contact ofrelay 452 and front contact of relay 4!! for operating relays 4M, M5 and432. Relay 432 locks under control of relay closes a circuit from groundat the front contact of relay 43|, through the right front contact ofrelay 1532, for operatively energizing the right wind ing of relay 43H5,and connects the ground at the front Contact of relay tsl to conductorE1!) as a seizure signal. The operation of relay 435 disconnects batteryfrom the winding of relay 435, to prevent the operation of battery 435in series with relay 525.) of transmitter ST2, and connects round toconductor 541 to operate a busy indicator 549 associated with the lineTi 2. The closing of the outel` left-hand Contact of relay 4|5 rendersthe continued operation of this relay independent of relay 412; and theopening of the back Contact of relay 4|5 prevents the operation ofrelays 42| and 425 while relay M5 is operated. Relay i5 closes a circuitfor operating the slowto-release relay 4i Relay 4 i 9 short-circuits reysistor M3 to increase the current through the relay H5 and the windingsof relays and d2! 'for operatively energizing each of these windings;and, since relay 442 operates before relay @its has released, relay 4t2also closes circuits including the upper winding of relay 4|4, backcontact of relay H5 and right front contact of relay lo for operativelyenergizing the Winding ci relay 52% and the upper winding of cut-offrelay 559 oi the associated toll line circuit TLC2. At its leit contact,relay @2l short-circuits resistor die; and, at its right contact, relay2l connects the winding of relay Ll2l and the lower winding of relay 55uin parallel with the winding or relay M5. Relay 525 closes a circuit foroperatively energizing the left Winding of relay '225. Relay Q26 closesa holdin'y circuit for its winding in parallel with the upper winding ofrelay 551i) so that these relays rem-ain operated, after relay ilereleases, until relay H2 releases. Relay 55@ disconnects the talkingconductors Eel and 682 from coil M9 and bridges the repeater terminationconsisting of condenser 555 and resistor 552 across these conductors.When the signaling current of irequency f2 ends, relay il? releases,causing they release of relays 26, 2! and 55%. Relays dill and 550 areslow in releasing, the left winding oi relay rl being operativelyenergized for about .020 second by current charging condenser 423 afterrelay 425 releases. The release of relay 25 disconnects the winding ofrelay e2? and lower winding of relay 55! from the front contact of relay4l l; and the release of relay reconnects line Tl2 and coil 529 totalking convductors 65| and B32.

The aforementioned operation of relay 32, in response to the rst part ofthe seizure signal, that is, upon operation of relay @Il in response tothe impulse of current of frequency jl, connects ground at the frontcontact of the normally operated relay d3 l through conductor a backcontact of relay Si@ of outgoing trunk circuit 0T2, back contacts ofrelays EM and ll' of the two-Way trunk circuit TWT2 to the winding ofrelay M3. Relay G'loperates, connecting ground to conductor 623 to markthe line Ti2 busy in the banks of selectors having access thereto. Thusthe operation of relaysl i, @32 and M3 in response to the current offrequency fl is effective to mark the toll line Tl 2 busy in the tandemoiice without waiting for the current of frequency f2, thereby reducingthe time interval after seizure at one end before it is protectedagainst seizure at the other end. Relay @i3 closes a circuit foroperating relay @le in series with line relay i583 of the associatedtoll selector TS2; this circuit is traced from ground at the backcontact of relay 43| of signal receiver SR2, right front contact ofrelay :232, conductor Git, back contacts of relays eli! and Ble, Windingof relay 525, inner upper front contact of relay cl3, thence in simplexthrough the windings of retard coil @lill of outgoing trunk circuit 0T2,outer back contacts of relay 633, outer iront contacts of relay 673.conductors l and 652, back contacts of relay 692 of selector circuitTS2, through resistors 685 and @ist to resistors 687, thence through aback contact of relay Sill) to the Winding of relay $88. Relay 525closes a circuit through resistor i for holding relay 513 and closes acircuit for operating the slow-to-release relay G'fl'l. Relay Gl? closesa holding circuit for relay 613 and con nects ground to conductor E83 tohold the succeeding circuit units through which the desired connectionis extended in the tandem orlice until release is eiiected ashereinafter described.

When the calling operator operates the dial key (not shown) and dialsthe toll route number, and also the called local oiice and suhscribersnumber in case the called line is located in oce in which dialcontrolled selectors are provided for completing desired connections,relay MD is alternately released and reoperated under control of thedial (not shown), the dialing circuit including conductor M32 and theupper winding of relay Edil. The release and reoperation of relay la inresponse to each dial impulse causes a like release and reoperationofrelay Ztil. The release of relay 20E] in response to the first impulseof each digit dialed, causes the operation of relay 205V),shirt-circuits the winding of normally operated relay 2l5 and opens theshort circuit across the winding of relay 261. Since relay ZEE is slowin releasing, it does not release during the response of relay Zilli todial impulses. Relay 22| is held operated by its right winding and relay240 is operated by its upper winding, both of which windings are inparallel with the winding of relay 2%. The operation of relay 2t@connects coil 233 to coil 360 and the operation of relay 2Q?!disconnects ground from conductor 256 thereby starting cur'- rent offrequency f2 through coils 239 and S, the conductors of line Tl2 andthrough coil 490. Relay Ziil is slow in operating taking longer tooperate than the sum of the intervals of time required for operatingrelays 2&9 and 2i3 but less than the minimum dial impulse. Relay 2G!remains operated while all of the impulses in the train are beingreceived and until .l second after relay 2% reoperates at the end of thelast impulse of the train. Relay 2M! also closes the circuit foroperating the slow-to-operate relay 253. The operation of relay 213,about .020 second after the operation of relay 299, disconnects groundfrom conductor 235 and connects ground to conductor 23e thereby endingthe current of frequency f2 and causing current of irequency fl in coils239 and 3519, the conductors of line TS2, and coil dell. When relay 298reoperates at the Vend of the rst dial impulse of a train, relay 2&9releases and relay 26? operates. The operation of relay 253 closes thecircuit for operating relay 2ST and disconnects ground at its backcontact from conductor 235; but until relay ZtlY operates, thisconductor is connected through both back contacts of relay 25H, irontcontacts of relays 2M, 2l3 and 2&3 to ground at a iront contact of relayZIB. The release of relay Edil also connects ground to conductor 235opens the operating circuit of relay 2l3.

- The Winding of relay 2l3 remains energized for about .020 to .030second after relay 299 releases, while condenser 2l l is charging,thereby preventing the release of relay 2l3 before relay 2%? operates.When relay 2&1 operates, it disconnects ground from conductor 235 andconnects ground to conductor 235. The connection of ground to conductor235 ends the current oi frequency f l and starts current of frequency f2in coils 237, l and dill). When relay 2&9 again releases at thebeginning of the second dial impulse of the train, relay 29S reoperatesrelay 203 releases. When relay 2l3 reoperates, about .020 second afterrelay ZES reoperates, ground is disconnected from conductor 235, andconductor 23d is connected to ground at the back contact of relay 223;and, when reiay 2&7 releases, ccnductor 2315 is also connected throughboth back contacts of relay 2?? and front contacts of relays 2li! and228 to ground at a front Contact oi relay 2id, thereby again initiatingcurrent of frequency f2 and terminating current of frequency ,i2 throughcoils '325i and ll. When relay reoperates at the end of the secondimpulse, relays and 2M operate in succession and re lays 269 and ZESrelease in succession thereby termina-ting theV current of frequency f2and starting current of frequency fl through coils 239, 366 and 466.Each succeeding impulse in the train effects a like operation of relays266, 269, 2|3, 263 and 2|3 and the transmission of impulse signalsconsisting of current of frequency f2 followed by current of frequencyfl. When relay 266 remains operated at the end of the last impulse ofthe train, relay 26| again releases, and relays 263 and 261 release toterminate the current of frequency f2 which is transmitted at the end ofthe last impulse. The release ci" relay 26| causes the release of relay246 to reconnect talking conductors |6| and |62 to the line TI2. Theouter contacts of relays 26|, 261, 2| 3 and 2|8 are provided to preventa short false impulse of frequency fl in case relay 266 opens its baci:contact before relay 263 closes its back Contact in response toy therelease of relay 266 and to prevent a short false impulse of frequencyf2 in case relay 263 opens its back contact before relay 266 closes itsback contact;

that is to place control of digit impulses after the first impulse offrequency f2 in relays 261 and 2|3 alone.

In the signal receiver SR2, the impulse of current of frequency f2 ofeach dial impulse effects the operation and release of relay 4|2 and theirnpulse of current of frequency fl effects the operation and release ofrelay 4| relay 4 2 closes circuits, including the upper Wind ing ofrelay 4|4 and back contacts of relays 4|| and 4|5, for operating relays42| and 425; but, since relay 4|6 is normal, relay 426 does not operate.Winding of relay 42|. Relay 42| short-circuits resistor 4|3` andconnects the winding of relay 421 and lower winding of relay 556 inparallel with the winding of relay 4|5. When relay 4|2 releases andrelay 4|| operates, a circuit is closed through the lower winding ofrelay 4|4, back contact of relay 4|2 and front contact of relay 4| I,for operating relay 4|5. The release of relay 4|2 causes the release ofrelay 425 but relay 42| is held operated for about .020 second afterrelay 425 releases by current charging condenser 423. Since relay 4||operates before relay 42| releases, relays 421 and 556 are operated inparallel with relay 4|5. Relay 4 I5 closes a circuit for operating4relay 4|9 and connects the lower Winding of relay 4|4 to the frontcontact of relay 4| independent of the back contact of relay 4|2. Relay4|9 short-circuits resistor 4 3 and connects the windlng of relay 426 tothe back contact of relay 4|5. Relay 421 short-circuits the right frontcontact of relay 42| so as to be held operated after relay 42| releasesas long as relay 41| remains operated. Relay 421 opens the normallyclosed circuit through the winding of relay 43|, closes a circuit foroperatively energizing the left winding of relay 436, and short-circuitsthe winding of relay 433. Relay 433 is slow in releasing and remainsoperated while relay 421 is responding to dial impulses. Relay 43| isheld operated for about .020 second, after relay 421 operates, bycurrent charging condenser 429. When relay 43| releases, it disconnectsground from conductor 610. When relay 4| releases at the end of thecurrent of frequency fl, which is the end of the dial impulse signal,relays 4|5 and 421 release and the lower winding of relay 556 isdeenergized. The release of relay 421 opens the short circuit across thewinding of relay 433, deenergizes the left winding of relay 436 andcloses the circuit for The operation of A Relay 425 closes a circuitthrough the left llo operating'relay 43|. `The operation of relay 43| 75about .020 second after its operating circuit is closed, reconnectsground to conductor 610. Relay 4|2 is reoperated by the current offrequency f2 and relay 4|| is reoperated by the current of frequency flof each succeeding dial impulse signal incoming over line T|2 from thetransmitter STI thereby effecting the alternate disconnection of groundfrom conductor 616 and reconnection of ground to this conductor inresponse to each dial impulse. The current of frequency f2 following thelast impulse of a train, effects the reoperation of relays 4 2, 42 l,425 and 426 and holds relays 4|4 and 550 operated. When the current offrequency f2 ends, relays 4|2, 4|4, 42|, 425, 426 and 550 release and nofurther operation of the signal receiver occurs until the next signal isreceived.

The disconnection of ground from and reconnection of ground to conductor610 by the response of relay 43| to each dial impulse signal of thefirst train incoming over line T|2, causes the release and reoperationof relay 615 of two-way trunk circuit TWTZ and the release andreoperation of relay 688 of selector TS2. Being slow in releasing, relay611 remains operated during the response of relay 615 to dial impulses.`The response of relay 688 to dial impulses effects the operation ofselector TS2 to select a corresponding level; and. the brushes ofselector TS2 are thereupon automatically advanced in the selected levelto hunt for an idle set of terminals. When an idle set of terminals areengaged by the brushes, relay 690 is operated to disconnect relay 688from conductor 616 and connect this conductor through brushes 69|, 692to the line conductors of the trunk or selector connected to the seizedset of terminals. Reference may be had to the aforementioned King et al.patent for a complete O lo'c'luousr: ol .selector .nba and 'tSOperation.

Assuming the call to be one Which is routed through an operatorsposition in the tandem oce, the selector TS2 selects an idle set ofterminais such as the set 696 thereby .onnecting conductor 616 throughbrushes 69| an 62, conductors |06| and |662, and winding gretard coil|666 of incoming trunk circuit` IT2A to the winding of line relay |665.The operation of relay ltli, in series with relay 616 or' the two-waytrunk circuit TWTZ, lights an answering lamp (not shown) to signal theoperator. When the call is answered by insertion of a plug P3 in jack Ji0, the lamp is extinguished and'relay |634 operated in the mannerdescribed in the aforementioned King et al. patent. rThe operation ofrelay |634 connects ground to conductor |664, thence through brush 694 afront contact of relay 666, normally closed Contact of the eleventhrotary step springs 689, and through conductor 684, to the Winding ofrelay 61| of two-Way trunk circuit TWT2. Relay 61| operates, connectingbattery through the back Contact of relay 616, back contact of relay 646of outgoing trunk cir-` cuit OTZ and through conductor 664 to operaterelay 566 of the signal transmitting circuit ST2. At its back contact,relay 566 opens a normally closed short circuit across the winding ofrelay 5|8; and relay 5|8 operates Within .056 second, connecting theupper winding of relay 546, left winding of relay 52| and winding ofrelay 569, allinparalleLto the back contact of relay 566.A At its frontcontact, relay 566 closes a circuit for operatively energizing the rightwinding of relay 52|, closesacircuitfor operating relay 563, closes acircuit for operatively energizing the lower Winding of relay 540, andshort-circuits the winding of the current of frequency f3 through coils539 and normally operated relay 5E i. Relay @2i connects ground toconductor Eil? to maintain the operation of the busy indicator 55E-ll;but, since the ,relay 43e or the sigaar receiving circuit sas is0ptermination comprising condenser l and resistor 552 across the talkingconductores, and connects the left winding of coil 539 to and throughconductors 5M yand 5&2 to coil alle. Relay 55H is slow in releasing andremains operated for about .1 second after relay ill operates. Relay 563closes a circuit for operating relay 55? and disconnects ground fromconductor 35 to start llill, the conductors of line TIZ, and coil p linecircuit TLCi in the calling toll office. lay 5G? is slow in operating,the actuation of its contacts, about .020 second after its o eratingcircuit is closed, being effective to connect conductor 535 to ground atthe back contacter relay 5% and to disconnect ground from conductor 53S.When relay dit operates, conductor 535 is also connected through bothback contacts of relay EES, front contacts of relays i and Elli, toground at a front contact of relay The connection of ground to conductori535 ends the current of frequency f3 and the disconnection of groundfrom conductor 535 starts current of frequency ffl through coils 53'3and the conductors of line Tl2 and coil Si? When relay 591 releases, .lsecond after relay 5% operates,

vrelay 5% releases and the lower winding of relay El@ is deenergized.Relay @t3 opens the operating circuit of relay Sill but relay il'a' isheld for from .020 to .030 second by current charging condenser 5%. Therelease of relay Eile, about .020 second after the release of relayElli, disconnects coil 53S from coil llt@ and reconneots l1ne TI2through coil dil@ to the talking conductors SGI and E. The release oirelay Sill reeonnects conductor 5% to ground at the back contact ofrelay 5&9. The release of relay EQ?, or the release of relay Edil,whichever occurs iirst, ends the current of frequency ffl in coils 539and de, the conductors of line Ti and coil 3%. Thus an answeringsupervisory or Off-hook signal is transmitted over line Till by animpulse of eurrent of frequency f3 for about .020 second followed bycurrent of frequency fil for about .l second.

In signal receiver SRi, the signaling currents are passed through coils390 and 323, aniplier 3M, iilter 355, to the volume limiting amplifier336. The signaling energy of frequency ,f3 is passed through filter 3%?and amplier detector 359 to energize the winding of relay Sil; and theenergy of frequency fd is passed through filter 3&8 and amplifierdetector Eli) to energize the right Winding of relay Sit. The output ofamplifier 393% is limited to a value sufficient for operating relay Slionly when substantiallyall of the current is of frequency f3 andsurlicient for operating relay SI2 only when substantially all of thecurrent is of frequency fil, thereby preventing the false operation ofeither of these relays in response to noise, voice or other interferingcurrents having components of one or both of the signaling frequenciesf3 and fil. The operation of relay 3H, in response to current impulse offrequency f3 of the answering supervisory signal, closes a circuitincluding the lower winding of relay 3M, back Contact of relay SI2 andfront contact of relay 35i for operating relays Sie and ili. Since relay335 is operated, the operation of relay 3H does not cause the operationof relay S32. The closing of the outer right front Contact of relay SI5renders the continued operation of this relay independent of relay 3.!2;and the opening of the back contact of relay SI5 prevents the operationof relays @2i and 325 while relay SI5 remains operated. Relay 355 closesa circuit for operating relay Sill. Relay 359 short-circuits resistorSEE to increase the current through the biasing Winding of relay SI2.Relays 3H, SI5 and Sii) release in succession when the signaling Currentof frequency f3 ends, but relay 3m is held operated by current chargingcondenser Sli for about .O20 second after relay 3&5 releases. Theoperation of relay SI2, in response to signaling current of frequency f4closes circuits including the upper winding of relay Sie, back contactof relay 3! I, front Contact of relay SI2, back contact of relay SI5,and the windings of relays 325 and 32I for operatively energizing eachof these windings; and, since relay IQ has not released, relay 3i2 alsocloses circuits including the upper winding of relay 3M, back contact ofrelay all, front contact of relay Slg, back contact of relay Sie andleft front contact of relay 3l@ for operatively energizing the windingof relay S and the upper winding'of cut-01T relay 25d. At its rightContact, relay Sli short-circuits resistor EIS; and, at its leftcontact, relay @2i connects the winding of relay 327 and the lowerwinding of relay Zli in parallel with the winding of relay 355. Relay325 closes a circuit for operatively energizing the right winding ofrelay 32 I. Relay 326 closes a holding circuit for its winding inparallel with the upper winding of relay 25u so that these relays remainoperated, after relay 3I9 releases, until relay 3 I 2 releases. At itsleft front contact, relay 32% closes a circuit including the right frontcontact of relay 335 for operating relay 332. Relay 332 locks undercontrol of relay 333 and closes a circuit including the back contact lofrelay 3?I and left front Contact of relay 332 for operatively energizingthe left winding of relay 355-5. Relay 335i does not disconnect batteryfrom the Winding of relay 335, since relay 335 was operated upon seizureof outgoing trunk circuit OTI as hereinbefore described. Relay 250disconnects the talking conductors ISI and |62 from coil 33S and bridgescondenser 2:5! and resistor 252 across these conductors. When thesignaling current of frequency fl ceases, relay 392 releases, causingthe release of relays BEI, 325, 3.25 and 25). Relays 32! and V25@ are.slow in releasing, the energization ofthe right Winding of relay 32!being maintained for about .020 second after relay E25l releases .bycurrent charging condenser 323. The release' of relay 32| disconnectsthe winding of relay E27 from the front contact of relay Sli; and therelease of relay `25B reconnects line T12 and coil iill'to talkingconductors Il and M2.

The aforementioned operation of relay 332, in response to signalingcurrent or Yfrequency ill following signaling current of frequency f3,closes a circuit from ground at the front contact of normally operatedrelay Ml, Vthrough conductor Il, a front contact of relay Il@ ofoutgoing .trunk circuit OTI, and winding of relay HB5, to battery; RelayIE5 operates, disconnecting ground from' .the lower windingofrelay 'Iildto increase the resistance of. the circuit through conductor |03 andthereby effect the extinguishlng of the supervisory lamp (not shown)associated with plug P2 of cord CDl, in the manner set forth in detailin the aforementioned King et al. patent.

When dialing is completed, relay |30 is operated in the manner describedin the aforementioned King et al. patent. The talking connection betweenoutgoing trunk circuit OTI and incoming trunk circuit IT2 includesconductors |||I and |02, front contacts of relay |30, conductors IGI and|62, back contacts of relays 253 and 240 of toll line circuit TLCl,conductors 24| and 242, left windings of coil 393 and resistors 30| and302, repeating coil 300, line TI, repeating coil 400 of toll linecircuit TLC2, right windings of coil 403 and resistors 40| and 402,conductors 54| and 542, back contacts of relays 540 and 553, conductors36| and 652, back contacts of relay |530 of trunk circuit 0T2, frontcontacts of relay 613 of trunk circuit TWTB, conductors 68| and 582,front contacts of relay 533 of selector TS2, brushes 60| and E32,conductors IO0I and |002, condensers |350, and conductors I 04| and I042 of incoming trunk circuit ITE.

If the answering operator disconnects cord CD2 from jack JIO, relay |034is released in the manner described in the aforementioned application ofJ. G. Walsh to effect the sending of an on-hook signal to the callingoperator. The release of relay |034 disconnects ground from conductor|004 and eifects the release of relays 915 and 500. The release of relay500 opens the short circuit across the winding of relay 50|, closes ashort circuit across the Winding of relay 5|8, closes circuits foroperating relay 509, for holding relay 52| operated and for operativelyenergizing the upper winding of relay 540. Relay 540 connects coil 539to coil 400 and connects condenser 55| and resistor 552 in series acrossconductors 66| and 632. The operation of relay 509 closes the circuitthrough the winding of relay 5|3 and disconnects ground from conductor536 thereby starting current of frequency f4 through coils 533 and 400,line Tl2, and coil 300. About .020 second after relay 509 operates,relay 5|3 operates, i'

509, 52| and 540. The release of relay 533 opens the circuit of relay5|3 and connects ground to conductor 533. Relay 5|3 is held operated forabout .020 second while condenser 5|| is being charged, the release ofrelay 5|3 .being effective to reconnect ground to conductor 535 thusterminating current of frequency f3 through coil 539. The release ofrelay 540, about .020 second after relay 5|8 releases, disconnects coil539 from coil 400 and reoonnects line TI2 through coil 300 to conductors06| and 662. Thus a signal consisting of current of frequency f4followed by current of frequency f3 is transmitted over line TI2 to thereceiver SRI in response to the disconnection of cord CD2 from jack J l0. In receiver SRI, relay 3|2 operates in response to the current offrequency f4 thereby effecting the operation 0f relays 3| 4, 32| and325. The release of relay 3|2 at the end of the current of frequency f4causes the release of relays 32| and 325. The operation of relay 3| inresponse to the current of frequency f3, eects the operation of relays3I4, 3|5, and also the operation cf relays 321 and 250, since relay 32|is held operated for about .020 second after relay 325 releases bycurrent charging condenser 323. ing in `parallel with relay 3|5 so as toremain operated after relay 32| releases until relay 3II releases. Theoperation of relay 3|5 causes the operation of relay 3I9; and theoperation of relay 321 short-circuits the winding of relay 333 andcauses the release of relay 33|, the release of relay 33| being `delayedby -current charging condenser 329. Relay 33| disconnects ground fromconductor |10 to release relay |05 of trunk circuit OTI. Since thecurrent of frequency f3 continues for about 1.3 seconds, the operationof relay 321 causes the release of relay 333; and the release of relay333 causes the release of relay 332. When relay 3l| releases, at the endof the current `of frequency f3, relays 3|4, 325 and 321 release. Therelease of relay 3|5 causes the release of relay 3|3; and the release ofrelay 321 causes the release of relay 250 and reoperation of relay 33|.Since relay 332 has released, the recperation of relay 33| isineffective to reoperate relay |05; and, therefore, the supervisory lampassociated with `plug P2 is permanently lighted.

If the calling operator Wishes to recall the answering operator,operation of the ringing key, which is not shown but is associated withcord CDI, causes a momentary release of relay |40 in the mannerdescribed in the aforementioned King et al. patent. The release andreoperation of relay |40 causes a release and reoperation of relay 200,whereby the signal transmitter STI operates to send a signal consistingof current of i; frequency f2 followed by current of frequency f|followed :by current of frequency f2 over toll line T|2, the same as fora single dial impulse. The signal receiver SRI responds to the reringsignal in the manner hereinbefore described for responding to a dialimpulse thereby effecting a momentary opening and reclosure of thecircuit through conductor 610, causing the release and reoperation ofrelay 615 of two-way trunk circuit TWTZ and relay |005 of incoming trunkcircuit IT2. The release and reoperation of relay |005 causes theflashing of the supervisory lamp (not shown) associated with cord CD2until the answering operator answers the recall signal, all as describedin the aforementioned application of J. G. Walsh and patent to King etal.

If the answering operator Wishes to signal the calling operator,operation of the ringing key (not shown) of cord CD2 causes the openingof the circuit through conductor |004 in the manner described in theaforementioned application of J. G. Walsh, to cause the release andreoperation of relay 31| of two-way trunk circuit TWTZ and acorresponding release and reoperation of relay 500 of signal transmitterST2. The signal transmitter ST2 is thereupon operated to transmit aringinor signal consisting of current of frequency f4 followed bycurrent of frequency f3 followed .by current of frequency f4, in asimilar manner to that in which the signal transmitter STI effects thetransmission of a single dial impulse signal as hereinbefore described.The signal receiver SRI responds to the ringing signal in similar mannerto that in which the signal receiver SR2 responds to a dial impulsesignal, as hereinbefore described, thereby opening and clos- Relay 321connects its winding the circuit through conductor H to eect a momentaryrelease of relay it. The release of relay '|25 connects ground to thelower Winding of relay |64 to decrease the resistance in the circuitthrough conductor 23 and thereby effect the lighting of the supervisorylamp associated With plug P2 of cord CD! as described in theaforementioned King et al. patent.

If the selector TS2 encounters an all-trunksbusy condition in theselected group, closure of the eleventh rotary step springs connectsconductor 232 to interruptor thereby causing the alternate operation ofrelay 6H ior .5 second and release of relay 'll for .5 second until theconnection is released by the calling operator. Each operation andrelease of relay fili causes a corresponding operation and release ofrelay 563. The rst operation of relay 52S effects the transmission overline TiZ of a signal consisting of current of frequency f3 followed bycurrent of frequency f4, similar to the answering supervisory signal,hereinbefore described; and each succeeding release of relay 522 effectsthe transmission over line Ti2 of a signal consisting of current offrequency f4 followed by current of frequency f3, the operations of theVarious relays of transmitter ST2 being similar to those of thecorresponding relays of transmitter STi to effect the transmis-sion ofoff-hook and onhook signals as hereinbefore described. In receiver SR,each signal comprising current of frequency fil followed by current offrequency f4 effects operations similar to those effected in response toan off-hooi: signal and each signal comprising current of frequency ffl,followed by current of frequency f3, effects operations similar to thoseeffected in response to an ori-hook signal as above described. Relay Eis thereby alternately operated and released to flash the supervisorylamp associated with plug P2.

When the calling operator disconnects plug P2 from jack J 2 to releasethe connection, relay E015 releases and causes the release of relayslill, |38 and I4!! as described in the aforementioned application of J.G. Walsh. The release of relay Mil causes the release of relay 222. Atits front contact relay 22E opens the short circuit across the Windingof relay 2u! and causes the deenergization of the left winding of relay22l. At its back contact, relay 223 closes circuits for operating relays2539 and 2513 and for holding relay 22i. Relay Ztl] operates to co-nnectcoil 23S to toll line T52 and relay 229 operates to disconnect groundfrom conductor 235 and thus start a current of frequency f2 throughcoils 239 and 3653, line Tl2, and coil il. Relay 229 closes the circuitfor operating relay 2.!3; and when relay Y2|3 operates, about .020second later, it disconnects ground from conductor 235 and connectsconductor 232 to ground, thereby ending the current of frequency f2 andstarting a current of frequency fi through coils 239, D and 4&0. Whenrelay 2Q! reoperates, within .050 second after relay 262 releases,ground at the inner left front contact of relay 245 is connected throughfront contacts of relays 2l3, 258 and 2M and back contacts of relay 22'!to conductor 236. About .6 second after relay 25S releases, relay 215releases due to the short-circuiting of its Winding at the back Contactof relay Zilli. The release of relay 2I5 causes the immediate release ofrelay 2l3 and closes a short circuit across the winding of relay EIS.The release of relay 255 disconnects ground from conductor 236 and therelease of `relay 2 L3 connects ground to conductor 235 thereby endingthe current of frequency fl and starting current of frequency f2 throughcoils 229, Edil and 322. About .1 second after relay 2 l 5 releases,relay 212 releases thereby connecting ground at the back contact ofrelay 252 through the front contact of relay 222, and outer back contactof relay 2l@ to short-circuit the Winding cf relay 2lb. rhe release ofrelay Zit causes the reoperation of relay 2 i 5 thereby effecting thereep-eration of relay 2l3. The reoperation of relay 2l5 does not causothe reoperation of relay 2te since the Winding of relay 2l6 isshort-circuited through its inner baci-z contact and the front contactof relay 22] to ground at the back contact of relay 220. The reoperationof relay ZEE connects ground to conductor 232 and disconnects groundfrom conductor 235 thereby ending the current of frequency f2 andstarting current of frequency fl through coils 239, 37:32 and 222. About1.3 seconds after the release of relay 2S closes a short circuit acrossthe Winding of relay 2 i8, relay 2li! releases, thereby causing therelease of relays 222 and 22! and the deenergization of the upperWinding of relay 22E). The release of relay 269 opens the circuitthrough the winding of relay 2M; and the release of relay 2M, about .G20second later, connects ground to conductor 235 to end the current offrequency fi. The release of relay 22! disconnects ground from conductor212i leading to the busy indicator 2639 and causes the release of relays222 and 33 The release of relay 222, about .020 second after relay Ziareleases, disconnects coil 239 from coil 32) and reconnects coil 3G53 totalking conductors itl and i552. Thus the release of relay 22B effectsthe transmission of a disconnect signal over line Ti2 consisting ofcurrent of frequency f2 for about .020 second, followed by current offrequency fl for about .6 second, followed by current of frequency f2for about .l second, followed by current of frequency ji for about 1.3second.

The signal receiver SR2 responds to the disconnect signal in thefollowing manner. The first current impulse of frequency f2 causes theoperation of relays M2, 252, @2i and Since neither of relays 59 or 52|are operated when the rst impulse of the disconnect signal is received,relay i2 is more sensitive than when dial impulses are being received sothat this relay is more certain to respond in case interfering currentsare present. When the rst impulse of frequency f2 ends, relay M2releases and relay i525 releases but relay 22! remains operated forabout .G20 second. The first current impulse of frequency fl causes theoperation of relays il l, 4M, M5, and- 552i. rlhe operation of relay M5causes the operation of relay lilo and the operation of relay i2? causesthe release of relay 23|. Relay 23! disconnects ground from conductor62B to release relay 6'55 of two-Way trunk circuit T`Wf2 and relay i225of incoming trunk circuit ITZZ. Since relay @i2 holds relays 1327operated and relay ist released for about .6 second, relay fill' of thetwo-Way trunk circuit TWT2 also releases disconnecting ground fromconductor E83 to cause the release of relay 592 of selector TS2 and thereturn of this selector to normal as described in the aforementionedKing et al. patent. Relay 433 releases about .5 second after its Windingis short-circuited by the operation of relay 21, thus causing therelease of relay 432. If the answering operator has disconnected cordCD2 from jack J Il) and relay 52| is normal, ground at the back contactAof relay 521 is connected through conductor 543 and the back contact ofrelay 433 to operate relay 439. Relay 439 closes a circuit for holdingrelay 436, lmaintains the short circuit across the Winding of relay 433,and closes a short circuit around the Winding of slow-to-release relay431. About .4 second later, relay 431 releases, causing the release ofrelay 439 and preventing its own reoperation as long as relays 433 and52| remain released. When the first impulse of current of frequency flends, relays 4I I, 4I5, 4I9 and 421 release. The release of -relay 421is ineffective to reoperate relay 433 While relay 439 is operated. Theoperation of relay 4|2 responsive to the second impulse of current offrequency f2 holds relays 4I4 and 559 and operates relays 42|, 425 and425. Since relay 435 is not operated, the operation of relay 425 isineffective to cause the reoperation of relay 432. When the secondimpulse of current of frequency f2 ends, relays 4I2, 42|, 425 and 426release. The operation of relay 4| I, in response to the second impulseof current of frequency fI and before relay 42| releases, holds relays4I4 and 559 and operates relays 4I5 and 421 but is ineffective to causethe reoperation of relay 432 because relay 4331s released. Relay 4|5closes the circuit for operating relay 4I9. The operation of relay 421causes the release of relay 43| and again connects ground to hold relay433 operated and relay 433 released. Since the aforementioned release ofrelay 439 does not occur until after relay 43| has reoperated, there isno opening of the short circuit around the winding of relay 433 untilthe second impulse of frequency fI has ended. When the second impulse offrequency fl ends, the release of relay 4| causes the release of relays4 I4, 4|5, 4I9, 421 and 559. The release of relay 421 causes thereoperation of relay 43|, the release of relay 433 and the reoperaticnof relay 433; and the operation of relay 433 causes the reoperation ofrelay 431. if the first impulses of current of frequencies f2 and fI ofthe disconnect signal should fail to effect release of relay 433, thesecond impulses will cause the release of relaysl 432 and 433, theoperation of relay 439 and release of relay 431. Since the nal impulseof current of frequency fl endures for about 1.3 second relay 431releases and relay 439 also releases before relay 421 releases. When thenal impulse ends, the release of relay 4II causes the release of relays4I4, 4I5, 4I9, 421 and 559; the

release of relay 421 causes the reoperation of rel lay 43|, the releaseof relay 436 and the reoperation of relay 433; and the operation ofrelay 433 causes the reoperation of relay 431.

From the foregoing description, it is t0 be noted that the busy relay22| of the signal transmitter STI at the calling end of toll line TI2 isoperated by the energization of one winding Whenever relay 209 isreleased until relay 2I8 reoperates after sending the disconnect signal;and busy relay 336 of the signal receiver SRI at the calling end of tollline T|2 is operated whenever a signal is being received. At the calledend of toll line T52,

the busy relay 436 of signal receiver SR2 operates as soon as relay 432operates in response to theseizure signal and remains operated until therelease of relay 421 when the last impulse of current of frequency fl ofthe disconnect signal ends; and the busy relay 52| of signal transmitterST2 is operated Whenever a signal is being transmitted to the callingoperator.

The signal receiver SR2 may be arranged to respond to a prolongedimpulse of current of frequency fl without being preceded by an impulseof current of frequency f2 by disconnecting conductor 434 from the rightfront contact of relay 421 and connecting this conductor to anotherfront contact of relay 4|5; which Contact connects ground to conductor434 whenever relay 4 I 5 is operated. When so modified, -the continuedoperation of relay 4I5 in response to current of frequency fI will shortcircuit the Winding of relay 433 to eiect the release of relays 433 and432 and thereby open the circuit through conductcr 619 to cause releaseas above described. When so arranged the receiver SRI will respond toeither a single frequency or a sequence disconnect signal.

Assume neXt that the call initiated by the calling operator in the firsttoll oice is one which is routed through the tandem oflice to a thirdtoll office, the first digit dialed effecting the operation of selectorTS2 to select a level giving access to the called toll cnice. Theterminals 395 representV such a level in the bank of selector TS2; and,

upon selection ol' these terminals and operation of relay 693, theholding ground connected to conductor 333 is further connected through afront contact of relay 693brush 393, conductor 923, and through theinner lower back Contact of relay 313 of two-Way 'trunk circuit TWT3, tothe winding of relay 914. Relay 914 operates and locks to conductor 923independent of relay 913. The aforementioned operation of relay 999 alsocloses a'circuit for holding relay 915 of two-Way trunk circuit TWT2 andfor operating relay 910 of two-way trunk circuit TWT3. This circuit istraced from ground at the front Contact of relay 43| of signal receiverSR2, front Contact of relay 432, conductor 919, back contact of relay@i9 of outgoing trunk circuit 0T2, back contact of relay 314 of two-waytrunk circuit TWT2, winding of relay 915, front contact of relay 313,windings of retard coil 339, outer back contacts of relay E39; outerfront contacts of relay 913, conductors 58| and 632, front contacts ofrelay 999, brushes 39| and `E592, conductors 92| and 922, outer backcon-L tacts of relay 913, outer back contacts of relay 939, windings ofretard coil 99), inner upper back contact of relay 913, through theupper operating winding of relay 910 to battery. Relay 919 closes acircuit for operating relay 833 of signal transmitter ST3 associatedwith toll line T34 leading to the called toll oliice. Thesignal'transrnitter ST3 is thereupon operated to send over toll line T34a seizure signal consisting of current of fre-V quency fI for about .G20second followed by current of frequency f2 for about .1 second. When thecalling operator dials the remaining digits of the called toll route andcalled subscribers line, the signal transmitter ST3 will be operatedunder the control of relay 43| of signal receiver SR2 and relay 919 oftwo-way trunk circuit TWTB to send dial impulse signals over line T34 insimilar manner to that in which the signal transmitter STI operates totransmit these impulses over trunk TI2 as hereinbefore described. Thereceiving cut-oif relay 559 of toll line circuit TLC2 and the sendingcut-olf relay 849 of toll line TLC3 are operated to prevent thetransmission of the dial impulse signals consisting of currents offrequencies f2 and JI through the talking connection between toll linesT12 and T34.

When an answering supervisory signal consisting of current of frequencyf3 followed by current of frequency f2 is received over toll line T34,the signal receiver SR3 responds in like manner to .that in which thesignal receiver SRI responds to such a signal as hereinbeforeolescribed. The receiving cut-off relay 856 is operated to prevent thetransmission of the answering signal through the talking connectionbetween toll lines T32 and T12. The operation of relay 732 -of signalreceiver SR2 in response to the answering supervisory signal connectsground to conductor 919, thence through a back contact of relay 91B ofoutgoing trunk circuit GT3, the lower front contact of relay Sift,conductor 922, brush 694 of selector TS2, front contact of relay ESB,back contact of springs @Bf-3, conductor 63a, to the winding of relaySil of two-way trunk circuit TWTZ. Relay S711 closes a circuit throughconductor 861i for operating relay 502 of the signal transmitter ST2 andan answering signal is thereupon transmitted through toll line T12 tothe signal receiver SR1 in the manner hereinbefore described.

When the answering operator in the third toll ofce or called subscriberreleases the connection, y

the signal receiver SR3 receives an on-hook signal consisting of currentof frequency f4 followed `by current of frequency f3, whereby relay '131is momentarily operated to disconnect ground from conductor 91! and thuscause the release and reoperation of relays 511 and `5&111 insuccession. The release and reoperation of relay 52S causes the signaltransmitter ST2 to send over toll line T12 a signal consisting ofcurrent of frequency fil followed by current of frequency f3. Theoperation of receiving cut-off relay 851) of toll line circuit TLC3prevents the on-hook sivnal from being transmitted through the talkingconnection between lines T314 and T12.

Ringing signals received over toll line T34 by receiver SRS are repeatedover toll line T12 to the signal receiver SR1 in similar manner to thatin which on-hook signals are repeated as above described; and recallsignals sent over toll line T12 by the signal transmitter STI arereceived by signal receiver SR2 and repeated by relays 431, 91) and 82)to effect the operation of the signal transmitter ST3 to send a likesignal over toll line T34.

When the calling operator disconnects, a disconnect signal is sent bythe transmitter ST1 over toll line T12 to `the receiver SR2. Thereceiving cut-off relay y553 operates to prevent the transmission ofthis signal through the talking connection between lines T12 and T314.The disand 9M of two-way trunk circuit TW I3 and the i release of relay829 of signal transmitter ST3. The release of relay 820 causes thesignal transmitter ST3 to operate to send a disconnect signal over tollline T34 to the signal receiver associated with toll line T34 in thethird toll oce.

It should here be noted that on calls routed through the tandem office,all of the first frequency and a part of the second frequency of asignal in either direction is passed through the talking connectionbetween lines T22 and T34 before the cut-off relays operate to open thetalking connection. Referring to the signal receiver SRI a false signaloperation in response to the part of a signal passed through the tandemofce from toll line T34 to toll line T12 before the talking connectionis opened, is prevented by making the signal relay 331 slow inreleasing. Relay 314 operates, whenever either one of relays 31 1 or 312operates, to disconnect the ground at its back contact from the directcurrent signal receiving circuit through conductor i'l leaving thiscircuit under the control of relay 331. But, since relay 331 does notrelease until about .020 second after relay 32'! operates, relay 3M willhave released due to the current of frequency f1 being ended by theopening of the talking connection in the tandem oflice, and theconnection of ground through the back contact of relay 315, to conductor112 prevents a false signal operation. To prevent impulse distortionduring the receipt of dial impulses on an incoming call, relay 331 isalso slow in operating so that the closing of the direct currentsignaling circuit through conductor 1li! is delayed for the sameinterval that the opening of this circuit is delayed. In case there issufficient distortion of the impulses in the voice frequency signalingcircuits to require impulse correction, an impulse connecting repeatermay be substituted for relay '31, the delay inherent in such a repeaterbeing effective to prevent false operation in response to partialsignals passed through the talking connection in a tandem office beforethis connection is opened.

Calls incoming over toll line T34 to the tandem oiiice are completed toan answering operator in the tandem oice or to a toll line T12 to thefirst toll ofce in similar manner to that in which like callsoriginating in the first toll office are completed, as hereinbeforedescribed. The lines T12 and Ta may also be seized by an operator in thetandem cnice on outgoing calls in which case the associated outgoingtrunk, two-way trunk, signal transmitter and signal receiver circuitsfunction in similar manner to that of the corresponding circuits oncalls from an operator in the rst toll oiice as hereinbefore described.

When a common volume limiting amplifier is used for both signalingfrequencies, as in signal receiver SRE and SR2 above described, thelimiter aids in preventing a signal operation in response to voice andnoise currents and is also effective at the time the signaling currentis changed from one signaling frequency to the other to aid the releaseof the relay operated by the rst frequency. 1f, however, echoes of thefirst frequency are received at a relatively high level with respect tothe signaling current of the second frequency, the relay responsive tothe first frequency may be held operated and the operation of the relayresponsive to the second frequency may be delayed long enough to preventa signal response. A high level of noise may also cause failure torespond to either or both of the currents which constitute a signal. Onemethod of avoiding these diificulties is to provide separate volumelimiting amplifiers for each frequency as shown in Fig. 10. In thisfigure the toll line T12 is shown with signal receiver SR1 and SR2connected thereto, these receivers being similar to the signal receiversSR1 and SR2 of Figs. 3 and 4 except that separate volume limitingamplifiers 3DS', 312', 4139 and 412 are provided. The band-pass filters30'! and 368 and 401 and 428 are the same as those of like designationin receivers SRi and SR2; and the signal relays 3.11, 312, A1 and 212and the other relays (not shown) are identical with corresponding relaysin receivers SR1 and SR2. Each of the amplifiers 32S', $022129 and Milmay be of the type disclosed in the patent to N. Bishop No. 1,863,895 ofJune 2l, 1932, and in the patent to G. T. Lorance No. 1,654,085 ofDecember 27, 1927, or of the type disclosed in the patent to E. PetersonNo. 1,830,240 of November 3, 1931.

Referring to the receiver SRE', the band-pass

